1. Field of the Invention
The present invention relates to an impregnated cathode comprising a porous element of a refractory metal and so on impregnated with an electron emitting substance (emitter) such as barium oxide (BaO) and a method of manufacturing the same, an electron gun and an electron tube.
2. Description of the Related Art
An impregnated cathode is used for an electron gun of a cathode-ray tube such as a picture tube and a display tube or an electron gun of an electron tube such as an image pickup tube and a high-frequency oscillator tube. Electrons (thermoelectrons) are emitted from the impregnated cathode.
The factors that determine the performance of such an impregnated cathode include a cathode cutoff voltage characteristic and a grid emission characteristic. It is important to reduce variations in the cathode cutoff voltage. The cathode cutoff voltage depends on the distance between the cathode and the first grid, the distance between the first and second grids, the thickness of the first and second grids, the aperture diameter of the first and second grids and so on. The grid emission is a symptom in which unintended emission of electrons occurs from excess barium and the like deposited on the grids (G1, G2 and so on). The grid emission is thus required to be reduced. In order to suppress unintended emission of electrons while maintaining the cathode cutoff voltage characteristic, it is required to increase the porosity in the electron emission region (working area) of the surface of the sintered porous element making up the impregnated cathode. At the same time, it is required to reduce the porosity rate or eliminate the pores in the other region so as to prevent the electron emitting substance for impregnation from being excessively vaporized through the region other than the electron emitting region.
Related-art impregnated cathodes are largely categorized into those of a single structure and those of a dual structure. The single-structure cathode only consists of a sintered porous element made of a refractory metal such as tungsten (W). The dual-structure cathode (such as the one disclosed in Japanese Patent Application Laid-open Sho 60-62034 [1985]) includes the electron emission region made of a porous sintered body and the region surrounding the electron emitting region made of a nonporous refractory metal. The two regions are fixed to each other through welding, for example.
However, such related-art impregnated cathodes have the following problems. It is difficult to make desired local variations of the porosity of the single-structure cathode made of a sintered porous element only. It is therefore extremely difficult to obtain the impregnated cathode whose impregnation amount of electron emitting substance is controlled as desired. If the cathode is impregnated with an ample amount of electron emitting substance so as to achieve a stable electron emission characteristic, barium (Ba) or barium oxide (BaO) as an electron emitting substance may evaporate and deposit onto the first or second grid during an operation of the cathode. As a result, the distance between the cathode and the first grid, and the distance between the first and second grids change and the cathode cutoff voltage drifts. Furthermore, it is impossible to reduce the grid emission.
On the other hand, in the dual-structure cathode made of a sintered porous element and a nonporous refractory metal, the refractory metal does not function as a storage of the electron emitting substance since the refractory metal is not capable of being impregnated with the electron emitting substance. Consequently, it is impossible to keep a sufficient amount of electron emission substance in the cathode for achieving a stable electron emission characteristic. The electron emission characteristic is thereby reduced and the life of the cathode is shortened. Another problem is that the manufacturing process is complicated and costs rise.